Gong-kuang zidonghua (Dec 2022)
High-efficiency gas extraction technology of staged fracturing roof with sand of underground broken and soft coal seam
Abstract
The gas extraction method of floor cross-layer drilling commonly used in gas control of broken and soft coal seam has problems such as large excavation quantity, long control period, short coal uncovering section of drilling, limited extraction control effect and so on. The gas extraction method of bedding short hole has problems such as poor drill-forming property, short extraction drilling hole, small extraction area and so on. This paper makes statistics on mechanical parameters and in-situ stress of coal seams and their roof and floor surrounding rocks in five typical broken and soft coal seam mining areas in Huaibei, Huainan, Jiaozuo, Jincheng and Yangquan. It is concluded that the elastic modulus of roof rock is 2.56-6.71 times of that of the broken and soft coal seam. The Poisson's ratio is 0.48-0.84 times of that of the coal seam. The analysis shows that roof rock of broken and soft coal seam is characterized by high elastic modulus and low Poisson's ratio. The roof is easier to be fractured than broken and soft coal seam. Referring to the idea of staged fracturing roof with sand of the horizontal borehole of surface coalbed methane, the idea of gas extraction by staged fracturing roof with sand of underground broken and soft coal seam is put forward. The directional long boreholes in stable strata of coal seam roof are constructed. The boreholes are generally less than 10 m away from the coal seam. The sand-carrying fracturing shall be carried out from the inside to the outside section by section in the boreholes. It will form a multi-stage fracture network in which the rock layer fully connects through directional long holes and the coal seam fully connects through fracture network in coal strata. The proppant is used to ensure that the fracturing network is in open state, so as to realize efficient gas extraction in a large area by the directional long hole in roof of broken and soft coal seam. The geological model of fracturing roof with sand in No.3 coal seam of a working face in Shanxi Xinjing Coal Industry Co., Ltd. is established. The numerical simulation of hydraulic fracturing coal seam and roof with sand is carried out by using FracproPT software. The result shows that the fracturing cracks in roof mainly extend to the coal seam in vertical direction. The length of fracturing cracks in roof in horizontal direction is 3.49 times of that of fracturing cracks in coal seam. This result shows that indirectly fracturing roof of broken and soft coal seam is better than directly fracturing the coal seam. Two 609 m directional long boreholes are drilled in roof of the No.3 coal seam in the working face to carry out gas extraction engineering application test of hydraulic staged fracturing with sand. The fracturing influence radius of the two boreholes is 20-38 m. The gas extraction pure amount of the fracturing holes is 1 025.11 m3/d and 2 810.60 m3/d respectively. The gas extraction pure amount of 100 m is 5.6-15.4 times of that of bedding un-fracturing boreholes in the same area. This study realizes high-efficiency gas extraction in a large area of broken and soft coal seam.
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